Lagrangian Analysis of the Meridional Overturning Circulation in an Idealized Ocean Basin

Abstract

The Lagrangian ocean model is used as a tool to simulate the response of the basin-scale overturning circulation to spatially variable diapycnal mixing in an idealized ocean basin. The model explicitly calculates the positions, velocities, and tracer properties of water parcels. Owing to its Lagrangian formulation, numerical diffusion is completely eliminated and water parcel pathways and water mass ages can be quantified within the framework of the discrete, advective transit time distribution. To illustrate the ventilation pathways, simulated trajectories were tracked backward in time from the interior ocean to the surface mixed layer where the water parcel was last in contact with the atmosphere. This new diagnostic has been applied to examine the response of the meridional overturning circulation to highly localized diapycnal mixing through sensitivity experiments. In particular, the focus is on three simulations: the first holds vertical diffusivity uniform; in the second, the vertical diffusivity is confined within an equatorial box; and the third simulation has a diffusivity pattern based on idealized hurricane-induced mixing. Domain-integrated deep ventilation rates and heat transport are similar between the first two cases. However, locally enhanced mixing yields about 30% younger water mass age in the tropical thermocline due to intense localized upwelling. In the third simulation, a slower ventilation rate of deep waters is found to be due to the lack of abyssal mixing. These results are interpreted using the classical theories of abyssal circulation, highlighting the strong sensitivity of the ventilation pathways to the spatial distribution of diapycnal mixing.